Repair Help and Education

Rear Derailleur Hanger Alignment

Useful Tools and Supplies

This article will discuss derailleur hanger alignment and use of the Derailleur Alignment Gauge DAG-2.

A misaligned derailleur hanger will often result in poor shifting performance. A common cause of a misaligned hanger is from the bike falling over to the right side. This pushes the derailelleur body inward, bending the hanger.

It is often possible to repeatedly re-bend many derailleur hangers. This is because there is very little stress from riding the bike or shifting gears. As a rule of thumb, if a hanger survives a repair by bending, it will survive use. However, there are some hangers that do not repair well. Extremely thick hangers and titanium hangers are difficult and sometimes impossible to repair. Bolt-on type hangers that are replaceable are alignable. However, these types of hangers can be tricky to align depending upon the frame hanger-to-dropout mounting design. Newly installed replaceable hangers should always be checked for correct alignment.

After aligning and correcting the derailleur hanger, it will be necessary to check all rear derailleur adjustment, including limit screw and index settings. For rear derailleur adjustments see Rear Derailleur Adjustment.

Procedure for Derailleur Hanger Alignment

Note: Newer DAG-2 models will now come with 2 o-rings for the slider. These will help prevent slider from falling out and can be used as markers when racks or fenders prevent full DAG-2 rotation. This procedure will be described at the end of this article.

Begin by mounting the bike in a repair stand with the wheels level as the bike would appear on flat ground. Check that the rear wheel that is mounted straight in the frame. The wheel does not need to be dished or true for use of the tool. Remove rear derailleur. Install DAG-2 and tighten handle. NOTE: Do not use the DAG-2 threads as a "chaser" of bad derailleur hanger threads. Chase and clean the threads using the TAP-10.

Rotate the arm toward the left side of rim, at the "9:00 o'clock" position. Rotate the tire valve to the 9:00 position. Use this point on the rim as a constant reference when checking the hanger. By checking the same point on the rim, wheel trueness or dish will not affect alignment.

Loosen the sliding gauger knob and move the sliding gauge to contact rim and secure knob.

It is easier to determine the error by seeing the gap between rim and gauge. In condition C, re-set tool at the 9:00 position and rotate back to the 3:00 position. There will be a gap between rim and gauge.

When bending hanger, it is best to bend small in amounts and re-check. The amount of error is actually one-half the gap between gauge and rim. As the gap is closed, it increases at the reference point 180 degrees away. Bend a bit, re-check both side, and then re-bend a bit more. Generally, it is best by having the DAG-2 arm next to the chainstay. This allows you to use the stay for leverage and control the amount of bending either inward or out. Repeat bending and checking until the gap is less then 4mm. Use a 4mm hex wrench as a "go-no go" gauge.

After getting the horizontal aligned, check and aligned, move on to check the 6:00 and 12:00 position. Set gauge to 6:00 position, then check at 12:00 position.

Again, bend only one-half the amount of gap. Reset pointer at each bending of hanger. When gap is less then 4mm, keep same the setting and check 3:00 position. If three points 90 degrees apart are within 4mm, hanger is aligned. Continue aligning as necessary.

Rack, Fender, or Other Interference

There may be fenders, racks or other situations where it the gauge is blocked from rotating to the various points. The newer DAG-2 models use two o-rings to hold the gauge in place. This helps prevent loosing the gauge. Additionally, the o-rings can be used as markers, allowing the gauge position to be accurately referenced, the gauge moved to pass the obstacle, and then the gauge returned to its original position. Below is an example of a bike with a rack the prevents rotation.

Push outer o-ring to slider. This marks the position of the gauge relative to the slider.

When the gauge cannot pass by a frame stay, fender stay, or rack, loosen gauge knob and pull gauge back to clear.

Return the gauge so the o-ring is back in the original position. In the image below, the gauge is contacting the rim, but the o-ring is positioned away from the slider. In this situation, the gauge would be resting inside the rim. There will be situations where the o-ring contacts the slider, but there will be a gap between rim and gauge end.